The present invention relates to a process for the preparation of chlorine from a gas mixture containing hydrogen chloride and further secondary components, such as sulfur compounds. The hydrogen chloride employed contains less than 100 ppm, preferably less than 50 ppm, more preferably less than 5 ppm, most preferably less than 1 ppm of sulfur in elemental or bonded form, based on the weight of the gas mixture.
A large number of chemical processes which require chlorine or phosgene as a reactant, such as the preparation of isocyanates or chlorination of aromatics, lead to an unavoidable production of hydrogen chloride. As a rule, this hydrogen chloride is converted back into chlorine by electrolysis (See, e.g., WO 97 24320 A1). Compared with this very energy-intensive method, thermal oxidation of hydrogen chloride with pure oxygen or an oxygen-containing gas over heterogeneous catalysts (the so-called Deacon process) according to4HCl+O22Cl2+2H2Ooffers significant advantages with respect to energy consumption (See, e.g., WO-A-04/0 14 845).
Catalytic oxidation of HCl gas with O2 to give Cl2 and H2O is typically carried out over heterogeneous catalysts. A variety of catalysts, e.g., catalysts based on ruthenium, chromium, copper etc., supported or unsupported may be used. Such catalysts are described, for example, in JP 2001 019405, DE 1 567 788 A1, EP 251 731 A2, EP 936 184 A2, EP 761 593 A1, EP 711 599 A1 and DE 102 50 313 A1. In particular, catalysts based on metallic ruthenium, ruthenium oxide, ruthenium mixed oxide, ruthenium oxychloride and ruthenium chloride, supported or unsupported, can be used. Suitable supports are e.g. tin oxide, aluminum oxide, silicon oxide, aluminum-silicon mixed oxides, zeolites, oxides and mixed oxides (e.g. of titanium, zirconium, vanadium, aluminum, silicon etc.), metal sulfates, alumina. The choice of possible supports is not, however, limited to this list.
It has now been found that sulfur components, such as H2SO4, SO2, SO3, H2S or COS, act as catalyst poisons. These sulfur components are gradually deposited slowly over the entire catalyst. The catalytic activity is reduced as a result. This reduced activity is unacceptable for use on a large industrial scale. The loss in activity of the catalyst can be permanent or temporary and reversible or irreversible. A further reason for the loss in activity is that most Deacon catalysts are thiophilic and therefore form more or less stable compounds with the sulfur even under very clean conditions thus rendering the catalytically active component inaccessible or deactivating it. For an optimum operation of the Deacon process, the lowest possible content of sulfur components in the HCl gas is therefore necessary.
In most processes, such as the preparation of isocyanates by phosgenation, however, considerable amount of sulfur components can be contained as an impurity in the HCl waste gas and introduced into the Deacon process. The sulfur components can have their origin in the natural gas/coal employed for the preparation of phosgene, i.e., introduction of the sulfur components into the Deacon process can take place via the CO quality in the preparation of phosgene and subsequently via the HCl process gas of the isocyanate installation. Further sources of sulfur can be present in the isocyanate process and pollute the HCl gas stream for the HCl oxidation. These additional sources of sulfur could have their origin in additives used in the isocyanate process (e.g., in the distillation), in the quality of the catalysts used (e.g., for the preparation and destruction of phosgene), and in the purity of the solvents used.
Since even the smallest amounts of sulfur can damage the Deacon catalyst reversibly or irreversibly, an expensive and comprehensive purification of the gas is to be undertaken before the isocyanate process gas comes into contact with the Deacon catalyst. This purification of the educts before they enter into the Deacon reactor is therefore essential for the life of the catalyst and accordingly for the profitability of the preparation of chlorine from HCl by catalytic oxidation.
EP 0 478 744 describes the adsorptive removal of SO2 from waste gases. A purification of an HCl stream for use in a Deacon process is not described here.
JP 2005-177614 describes the removal of sulfur components from gas containing HCl or Cl2. The removal is effected by bringing these gases into contact with metals or compounds thereof. The metals are chosen from groups 8-10 of the Periodic Table of the Elements.